The invention relates to medicament delivery devices. More particularly, it relates to an injection device, such as an auto-injector, for injecting a fluid medicament, wherein after the medicament has been delivered, a needle protecting sleeve associated with the injection device is moved or slid to cover the injection needle.
Moving or sliding a needle protecting sleeve in a proximal direction, from a distal position in which the needle is exposed, by using a spring, such that the needle is covered and protected against being accessed, is known from WO2002047746A1. The spring which forces the needle protecting sleeve to move or slide over the injection needle is pre-tensioned by an amount. When the needle protecting sleeve is moved or slid beyond the injection needle, the spring force decreases according to the rules of Hooke's Law. Thus, the spring has to be pre-tensioned such that the needle protecting sleeve can reliably assume an end position in which it fully covers the injection needle. However, a problem is that having a device with pre-tensioned springs acting on plastic components in storage for long periods of time may deform the plastic components. Also, having a device that is triggered or partially triggered by shifting the needle protecting sleeve from a proximal position into a distal position the spring is further tensioned by a further amount. A further problem is to obtain a balance of forces since it is desirable that the force needed to shift the needle protecting sleeve in the distal direction is low, to make it easier for the user to handle the device and on the other hand, an increased spring force is desirable for shifting the needle protecting sleeve in the proximal direction from the distal position.